Discussion on setting calculation of loss-of-excitation protection for large turbogenerator

Loss of excitation (LOE) is a common fault found in the generators. The most widely applied method for detecting a generator loss of excitation is the use of impedance relays to sense the variation of impedance as viewed from the generator terminals. However, compared with China\´s approachs, in the setting of impedance circle, the IEEE\´s allow the circle to extend along the negative X axis, meanwhile, in the latest edition of the "IEEE Guide for AC Generator Protection", the circle is further extended, reaching to -1.25X_d. We think this is adverse, to prove our point, the paper, basing on the IEEE 9 bus model and using computer digital simulation, have studied various loss-of-excitation conditions and some system disturbances that may lead to the relay false tripping. Simulation results indicate that the IEEE\´s method not only can\´t accelerate the operation effectively, but also will increase the chances of maloperation.